Abstract
Nowadays, triple phase ferrite-bainite-martensite steels have found widespread applications in automotive industry. The volume fraction of different phases is the key factor in determining mechanical properties of these steels. This paper investigates the effect of bainite volume fraction on microstructure and mechanical properties of a triple phase steel. Hence, an intercritical annealing treatment at 780 °C was followed by austempering treatment at 400 °C for different times. Microstructures were analysed using optical and scanning electron microscopy, and mechanical properties were also investigated by a tensile test. The results showed that the kinetics of austempering treatment had a good adaptation with JMAK equation. It was also found that increment of bainite phase from 0 to 42 vol.% decreased ultimate tensile strength from 952 to 578 MPa, but from 5.7 to 12 vol.% ductility increased. Results also showed that triple phase steels with 14.9 and 24.5 vol.% bainite demonstrated three-stage work hardening behaviour while other samples obeyed two-stage work hardening mechanism.

Key words
austempering treatment, triple phase steel, work hardening behaviour, mechanical properties

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